The invention relates to a system for monitoring current drawn from traction batteries in electric vehicles and hybrid vehicles.
German Patent Document DE-PS 33 17 834 discloses a control system for the traction motor of an electric vehicle with a traction battery, in which control system a set drawn current value (hereinafter referred to as "battery current") which is prescribed with the aid of an accelerator pedal is compared with two limit values and, if appropriate, limited in order to avoid overloading. The first limit value depends on the pulse duty factor of the power circuit breaker and, if appropriate, also on further operating parameters, for example, the temperature of the power circuit breaker, while the second limit value continuously decreases from a maximum value to a minimum value for the duration of the current limiting.
The disadvantage of this control system lies in the fact that, although the overload protection is activated as a function of the battery current, and not as a function of the temperature, damage to the battery is not caused by an excessively high battery current, but rather by the high temperatures which occur during this process. Under certain conditions, therefore, the battery current can be prematurely reduced; that is, even when dangerously high temperatures have not yet been reached. Taking into account a temperature-dependent factor during the calculation of the limit value cannot prevent this either.
The object of the invention is to provide a battery current monitoring system such that the short term availability of high battery currents is improved, while simultaneously ensuring an effective overload protection.
This object is achieved according to the invention by exploiting the fact that damage to the battery is not caused by a high battery current, but rather by the temperature rise associated therewith. Therefore, it is proposed not to start the battery current reduction until a critical battery temperature has been reached. As a result, the battery current is not reduced until the battery is actually at risk. The monitoring process itself continues to be started when a critical battery current is reached.
Since in a battery there is always a difference between the temperature prevailing inside the battery and the temperature which can be measured at its surface, it is proposed according to a further embodiment of the invention not to use the average measured temperature as the upper limit value but rather a battery temperature which is theoretically calculated on this basis.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.